1. Field of the Invention
The present invention relates to the field of power switch devices for rotating generators, preferably synchronous generators, with which high performance generators in a range of several 1 MW to 2000 MW can be connected to and/or disconnected from the grid within very short periods of time. Furthermore the present invention relates to methods for operating such devices. Generators and grids are polyphase alternating current systems. The grid is comprised of power consumers, power generators and connecting lines.
2. Brief Description of the Related Art
Typically a generator that is operated to generate electric energy is connected to a grid by means of a generator switch (between generator and transformer), a transformer, and a grid switch (between transformer and grid). It is known to turn on a generator switch when the three synchronization conditions of voltage phase sequence and voltage phase position, voltage amplitude, and slip are met. The connection is obtained by simultaneously closing the switches of each phase of the connection between generator and grid. For a customary three-phase system this means that each of the three phase binding posts of the generator is connected to the corresponding phase binding posts of the grid. It is irrelevant whether this occurs with one switch on generator voltage level (between generator and transformer, i.e., by means of the generator switch) or on high voltage level (between transformer and grid, i.e., by means of the grid switch).
Less complex power stations do not have any switches between the generator and the transformer. Instead, the grid switch is responsible for all functions. Smaller generators and supervoltage generators (Powerformer) are connected to the grid by means of a switch without any transformer.
If there are strong malfunctions in the grid, the generator is separated from the grid. This is achieved by opening the generator switch directly behind the generator binding posts or by opening the grid switch. If a sensor registers a fault in the grid, which typically manifests itself in a drop in voltage or a corresponding change in the current, such a power switch opens one or several of the three phases and thus prevents strong overcurrents. An example of such a malfunction is a short circuit in the line. Due to inertia of the output regulator of the turbine, the turbine generator unit that now is separated from the grid accelerates the number of revolutions in a first phase. This increases the generator frequency and the result is an increasing phase angle error as well as increasing slip between generator voltage and grid voltage.
The generator is automatically reconnected. Typically there is a period of 100 to 200 ms, 300 ms max. (a so-called “Critical Clearing Time”) during which the phase angle error and slip can still be tolerated. The dynamic compensation processes during the reconnect are barely acceptable and stability is barely ensured.
Typically, mechanical switches are used as power switches for this purpose. The dimensioning mainly is determined by the maximum disconnect performance that can be up to several 1000 MVA. The switch uses the periodic current crossovers for shutting down. Nonetheless so-called electric arcs occur when the contacts are disconnected. In order to be able to reduce these discharges in a controlled manner, the actual mechanical contacts usually are arranged in a switch box that is filled with an insulating gas such as SF6. ABB-Review 3/2002 (pp. 34-40; ABB, Baden, Switzerland) describes an example of such a design. This type of power switch can also be used to restore power by connecting live contacts.
EP 0984552 discloses the use of compensating resistance loads to expand the reconnection limits after grid malfunctions are removed. When the generator is disconnected from the grid, a resistance load that is equivalent to the preceding grid load is connected to the generator in order to minimize the development of a phase angle error as described. In particular, an adjustable resistance, for example in the form of a plurality of partial resistances that can be connected independent from each other, is proposed for the resistance load. The individual resistances are connected via thyristors that are arranged in an anti-parallel manner.